1. Field of the Invention
The present invention relates to electroetching of metal foils, films, or layers adhered onto an insulating substrate, particularly where the metal film is thin in comparison to the size of the etched features.
2. Description of the Prior Art
Etching of metal films is a process that is widely used in industry for manufacturing a wide range of products, from novelty items to computer chips. In many etching applications it is desirable to etch through a thin layer of metal laid on an insulating substrate or base layer; one of the more important commercial applications is microelectronics fabrication.
TAB (tape automated bonding) is an example of a microelectronic component, in which a metal foil adhering to a plastic sheet is masked with photoresist and then etched to remove bare metal. The mask material is then removed, leaving a pattern of conductive metal strips or lines on the sheet which act as wires. The sheet is aligned over a chip, and the lines make connection to various places on the chip.
Another example is a DLM (double layer metallurgy) package, in which layers of metal over a substrate are milled flat.
A third example is C4 (controlled collapse chip connection), an advanced chip packaging technique that employs tiny solder balls resting on pads. C4 requires a continuous seed layer for through-mask electrodeposition of the solder alloy. The seed layer is etched in between the solder pads after electrodeposition, leaving the seed layer under the isolated pads.
FIG. 1 illustrates the background art, showing a basic electroetching apparatus in accordance with the prior art. A tank T holds liquid electrolyte E, an aqueous solution of a salt (for example, table salt and water). The anode A and the cathode C are wired to a voltage source such as a battery B. When the apparatus is electrified, metal atoms in the anode A are ionized by the electricity and forced out of the metal into the electrolyte solution, so that the metal dissolves into the water. The rate of dissolution is proportional to the electric current, according to Faraday's law. Depending on the chemistry of the metals and salt, the metal ions from the cathode either plate the cathode, fall out as precipitate, or stay in solution.
Electrochemical machining (ECM) is based on the basic electroetching set-up of FIG. 1. In conventional ECM, the cathode is a shaped tool which is held close to the anode and slowly moved toward it. The anode is the workpiece, which is machined away as it dissolves.
In through-mask electrochemical micromachining, metal removal takes place at the exposed metal surface without any need to move the cathode towards the anode or maintain a close tolerance on the anode-cathode distance.
Referring to FIG. 2, an etched film 20 is shown adhered to a substrate 10. On top of the film 20 is a layer of photoresist 30, applied by photolithography or other conventional means. A contact 40 and contact lead 42 allow the metal layer 20 to be connected to a voltage source such as the battery B of FIG. 1, a power supply, or other conventional device.
The openings in the layer 30 existed before electroetching, but the metal layer 20 was continuous. Because of the etching, the metal film 20 in the openings of the mask has been dissolved away, so that the substrate 10 is seen in the gaps. In a larger region, an island 22 of the metal layer 20 remains behind, surrounded by bare substrate 10. Such islands are a problem in microelectronics fabrication.
The mechanism of island formation is shown in FIG. 3, a magnified portion of cross section 3--3 of FIG. 2 at a time just before the island 22 formed. In FIG. 3 the metal film 20 has been eroded away by electroetching. The bulge effect's characteristic spoon shape is seen in the metal 20 cross section. Due to faster etching near the edge of the mask 30, a moat has formed. The moat has a depth m at its lowest point, as compared to the shallow etched depth d near the center of the opening. As etching progresses, the depth m will increase until it equals the metal film thickness b, and then the island 22 seen in FIG. 2 will form. The metal of the island 22 is isolated from the rest of the film 20, and so no electric current can flow through it, and etching stops over the island 22.
FIG. 3 also depicts the undercutting of the bulge effect. The metal 20 has been eroded back a distance u under the edge of the mask layer 30.
According to the letters of FIG. 3, the etch factor is equal to: undercut/maximum depth etched (u/m); the moat factor is equal to: maximum depth etched/depth etched at center of opening (m/d).
It will be seen that no islands will form if there is only a narrow gap in the mask 30, but undercutting will still occur.
Chemical etching can be used to dissolve the metal, but ECM is preferable because it is faster, less dangerous, and less polluting.
Instead of strong acids, ECM uses an aqueous solution of a salt with some acid or base added. Such a solution does not by itself eat away at the metal film. But solutions of water and salt are electrically conductive, and if electricity is made to flow through the metal film while it is immersed in the solution, then the metal will dissolve into the solution. Metal atoms are ionized by applied voltage and then pulled out of the metal into an electrolyte. Usually, the electrolyte is sprayed or flowed over the surface to be etched to ensure uniform removal of material.
A substrate with layers of photoresist and etched metal is depicted in FIG. 2, labelled "prior art."
As etching progresses, metal is dissolved underneath the edge of the mask so that an undercut is typically formed.
An undercut is accompanied by a "moat," a dished-out region or trough running alongside a resist border. The undercut and the moat combine into a characteristic smooth, spoon-shaped curve, whose cross section is depicted in FIG. 3 (labelled "prior art"). The moat and the undercut together are called a "bulge."
The degree of undercut can be described by the absolute undercut (the width of resist left hanging over the metal edge) or by the "etch factor," defined as the ratio of the undercut to the maximum depth of the etch (u/m in FIG. 3). The moat can likewise be described by its depth or by the "moat factor," the ratio of the etch depth at the lowest point in the moat to the etch depth at the center of the opening (m/d in FIG. 3).
The undercuts and moats of the bulge effect are concerns in electronic fabrication. Manufacturers have tried to lower the undercut etch factor as much as possible to allow finer mask detail and to bring the moat etch factor under 1.0.
The resist and the resist lip (portion of resist extending over the undercut) are removed after the etching step is completed, but the undercut remains as a change in the size and shape of the metal pattern. Metal lines left between etched areas are narrowed. Because the undercut changes the shapes of corners and bends, and because the etch factor can vary from place to place, the undercut cannot be corrected by making allowances in the phototool design. Undercutting limits the breadth of lines and the clearance between etched circles, because if etched areas are set too close the undercuts will meet.
Moats cause a problem which is distinct from the problems caused by the undercut. Moats illustrate the problem of "islands". Islands are isolated areas of metal film surrounded by bare substrate. The moat is etched deeper than the metal at the center of the opening. The result is a metal island surrounded by the insulating substrate along the resist edge. The substrate is a non-conducting material, and the island metal is cut off from the rest of the metal film, so no electric current can flow to it.
Since electrochemical micromachining involves the use of neutral salt solutions and relies on the passage of an external current for metal removal, as soon as an island is formed, the electric current does not flow to the island and the island stops dissolving.
An island 22 is illustrated in FIG. 2, labelled "prior art."
Several mathematical models have been developed by researchers studying the bulge effect. These investigators have studied the chemical and hydrodynamic aspects of the problem, but not the electrical aspects.
For example, chemical engineering professors Richard C. Alkire of the University of Illinois, David B. Reiser of the same institution, and Robert L. Sani of the University of Colorado published a paper in the Journal of the Electrochemical Society concerning dissolution in small cavities (J. Electrochem. Soc. 131, 2795 (1984)). They used a computer to study how flowing electrolyte would swirl inside a cavity in a surface, for example, a depression being etched into a metal film over a circular gap in the resist.
Alkire, Reiser, and Sani concentrated their studies on this hydrodynamic aspect of the problem, modelling the fluid eddies and then finding the effect of the flow on the concentration of metal ions in the electrolyte. The ion concentration strongly affects the dissolution rate of the metal, because ionized metal atoms in the film more readily pass into an electrolyte which has a lower ion concentration; the ions are electrically charged and repel one another.
The authors did not consider ohmic resistance effects in their study, as they state in their paper (at the second paragraph in the second column on page 2796). That is, they did not take into account the electrical resistivity of the electrolyte as a bulk material, the current distribution in the electrolyte, or the electric field.
Another mathematical study was undertaken by H. K. Kuiken of the Philips Research Laboratories in the Netherlands, and reported in the journal of the Royal Society (Proc. R. Soc. Lond. A 392, 199-225 (1984)). Unlike the three-dimensional model of Alkire, Reiser, and Sani, Kuiken's study was two-dimensional, and so the results apply best to an elongated gully or ravine rather than to a circular hole. Like Alkire, Reiser, and Sani, Kuiken did not consider the electrical aspect of the problem, only diffusion (as they note in the Abstract at page 199).
Kuiken examined the effect of atomic diffusion in the case where the electrolyte is stationary instead of flowing. He generated a series of curves, which showed the characteristic spoon-shaped bulge with moat and undercut. Kuiken's work is summarized by Allen at page 124.
C. Vuik (with the Philips Research Laboratory and University Utrecht, Department of Mathematics, Utrecht, the Netherlands) along with C. Cuvelier (of the Department of Mathematics and Informatics, Delft University of Technology, Delft, the Netherlands) wrote on a numerical solution of the etching problem. Their work was published in the Journal of Computational Physics (J. Comp. Phys. 59, 247-63 (1985)). Like Kuiken, they considered diffusion but not electric fields or currents. In applying their equations they assumed that the resist layer was infinitely thin.
These theoretical studies have mathematically described some mechanisms of bulge problem, but have not solved the practical problems caused by it.
Another problem encountered in electroetching of thin metallic films atop insulating substrates is that of "contact resistance." The metal film can be very thin, as thin as a few hundred Angstroms. Such a thin layer has high electrical resistance. The contact resistance effect is an accelerating process because, as the film dissolves and becomes thinner, the contact resistance increases.
Contact resistance is not caused by the bulge effect at the mask edge. Rather, it is caused by the foil itself. When etching a continuous thin film without any mask, no islands would form but contact resistance would still be troublesome. The contact resistance problem is encountered in electroetching blanket as well as patterned films.
When electroetching in practiced with an apparatus similar to that shown in FIG. 1, the workpiece comprising the metal film atop an insulating substrate is immersed in liquid electrolyte E, in a tank T. The current for electroetching flows into the film at one or more contacts, and spreads out through the film. Contact resistance is the electrical resistance of the film, in ohms, as measured between any place on the film surface and the contact. The thinner the film the greater its resistance.
By Ohm's law, the current flowing through a resistive film causes a voltage drop across the film. If the whole area of the film were at the same voltage, then the current would be the same all over the film, and metal atoms would be driven from the metal into the electrolyte at the same rate everywhere. However, contact resistance causes different areas of the film surface to be at different voltages. In electroetching, the rate at which metal dissolves into the electrolyte at any one place on the film surface is proportional to the electric current density at that place, which in turn is a function of the voltage there. The voltage drops with increasing distance from the contact, so the metal will etch faster near the contacts. Eventually, the metal near the contacts is etched completely through before the metal farther away, the contacts are isolated, and there is no way to remove the remaining film of metal.
The converse of electroetching is electroplating, in which metal is added instead of removed. The voltage polarity across the electrodes is the reverse of the etching polarity, and a plating solution is used in place of the etching electrolyte. The converse of the contact resistance problem in plating is referred to as the "terminal effect."
The terminal effect causes a greater thickness of metal to be deposited near the contacts than farther away. As in etching, the voltage is higher near the contacts. The greater current caused by the higher voltage increases the rate of metal deposition near the contacts. Once enough thickness is built up, the terminal effect ceases. However, the uneven thickness is not eliminated, since subsequent plating is evenly distributed.